The present invention relates to a technique for minimizing the beam spot diameter of a charged particle beam apparatus. More specifically, the present invention relates to an aberration corrector with lenses in multiple stages including multipole lenses, which is used in a charged particle beam microscope apparatus including a scanning electron microscope.
In a charged-particle optical apparatus including electron microscopes such as a scanning electron microscope (SEM) and a transmission electron microscope (TEM), a lens using an electric field or a magnetic field to condense a charged particle beam is essential. In a rotational symmetrical lens generally used as the electromagnetic lens, chromatic aberration and aperture aberration such as spherical aberration inevitably occurs to define substantial resolution limit. Means preventing or correcting occurring aberration has been desired for a long time for making the resolution of the charged particle optical apparatus higher. An aberration corrector using a non-rotational symmetrical electromagnetic lens is one of radical and strong solving methods therefor. For instance, J. Zach, and M. Haider, Nucl. Inst. and Methods in Phys. Res. A vol. 363 (1995), p316–325 (Document 1) discloses an aberration corrector for SEM of a system having 12 poles in four stages.
FIG. 2 schematically shows the operating principle of the aberration corrector for SEM disclosed in Document 1. A corrector 1 has 12-pole lenses 2, 3, 4 and 5 in four stages. The 12-pole lenses 2 and 5 can form electro-static fields and the 12-pole lenses 3 and 4 can superimpose electro-static and magnetic fields, thereby generating a quadrupole field. Since each of the first to fourth stages has 12 poles, not only the quadrupole field but also a dipole/quadruple/hexapole/octapole field can be generated. The numeral 9 denotes an optical axis, that is, a charged particle trajectory incident upon the center axis of the corrector 1 at an angle of 0□,□ which is matched with the center axis of the corrector.
The quadrupole fields for an electric field and a magnetic field are expressed by the forms (1) and (2), respectively.
                              ϕ          2                =                                                            E                2                            ⁡                              (                z                )                                      2                    ⁢                      (                                          x                2                            -                              y                2                                      )                                              (        1        )                                          ψ          2                =                                            B              2                        ⁡                          (              z              )                                ⁢          xy                                    (        2        )            
Using z dependence portions of (1) and (2), the strength of the quadrupole field is defined by (3)
                              β          ⁡                      (            z            )                          =                                                                              E                  2                                ⁡                                  (                  z                  )                                                            Φ                0                                      +                                                            e                                      2                    ⁢                    m                                                  ⁢                                                      B                    2                                    ⁡                                      (                    z                    )                                                                                                          (        3        )            
to be indicated by the numeral 10. An axial potential about the optical axis 9 is indicated by the numeral 11.
The basic principle of aberration correction is in the use of a one-dimensional concave lens formed by a multipole field.
The correcting principle of the chromatic aberration in the corrector of FIG. 2 is described below. The corrector 1 can realize a one-dimensional convex lens having negative chromatic aberration by combining a one-dimensional concave lens by an electro-static field with a one-dimensional convex lens by a magnetic field in the second and third stages multipoles. The magnitude of the negative chromatic aberration can be arbitrarily adjusted by adjusting the ratio of electric and magnetic field so as to keep β(z) of the equation (3) constant.
To perform chromatic aberration correction using the one-dimensional lens, a charged particle beam passing through the corrector must select specific standard trajectories 7 and 8 satisfying the following trajectory conditions (i) to (iv).
(i) The x trajectory 7 passes through the center of the second-stage multipole lens 3.
(ii) The y trajectory 8 passes through the center of the third-stage multipole lens 4.
(iii) The off-axis distances of the x and y trajectories are equal in the outgoing plane of the fourth-stage multipole lens 5.
(iv) The slopes of the x and y trajectories are equal in the outgoing plane of the fourth-stage multipole lens 5.
A trajectory passing through the center of a lens is not subject to the effect of the lens including aberration. Using the standard trajectories 7 and 8 can isolate the x and y trajectories from each other to add negative suitable chromatic aberration at the second and third stages. As described above, the aberration corrector described in Document 1 suitably adjusts electro-static fields and electromagnetic fields generated by the 12-pole lenses in four stages to satisfy the four trajectory conditions for correcting chromatic aberration.